The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus which is structured in the manner that a light beam source is internally modulated corresponding to a pulse width modulation signal which is obtained corresponding to an image signal.
Generally, an image forming apparatus, such as a digital copying machine in which a laser beam is used, is composed of a scanner section 300, an image processing section 400 and a printer section 100 as shown in FIG. 5. (Refer to Japanese Patent Publication Open to Public Inspection No. 157070/1987, or the like.) The scanner section 300 optically scans documents 200, and image information of the documents 200 is converted into an optical image. The optical image is supplied to the image processing section 400 and converted into an image signal, and at the same time, a predetermined image processing operation is conducted.
The image processing operation includes variable magnification processing, halftone processing, and inside-void processing, and in the case of a color copying machine, color ghost processing.
In the printer section 100, an image is recorded according to a digital image signal (pixel data) having predetermined bits formed in the image processing section 400.
FIG. 6 shows an example of the printer section 100. In this example, an electrophotographic printer having a photoreceptor drum therein is used, and also a laser beam is used for a light source by which an electrostatic latent image is formed on the photoreceptor drum.
In FIG. 6, pixel data (DATA) outputted from the image processing section 400 is supplied to a modulation circuit 110. In the modulation circuit 110, a pulse width modulation signal (SPWM) is formed according to the pixel data (DATA).
The pulse width modulation signal (SPWM) formed in the modulation circuit 110 is supplied to a semiconductor laser 931 (light source) through a laser drive circuit 932, and modulates internally laser beams. The laser drive circuit 932 is controlled by a control signal outputted from a timing circuit 933 so that the laser is driven only within a horizontal and vertical effective area by the laser drive circuit 932.
A signal showing a light amount of laser beams is fed back from a semiconductor laser 931 to the laser drive circuit 932, and the semiconductor laser is controlled so that its light amount becomes constant.
Laser beams outputted from the semiconductor laser 931 are supplied to a polygonal mirror 935 and deflected. A starting point of scanning of the laser beams, deflected by the polygonal mirror 935, is detected by an index sensor 936. The detection signal from the index sensor 936 is converted into a voltage signal by a current/voltage conversion amplifier 937, and then an index signal SI is formed. The index signal is supplied to a control means by which optical scanning timing of the scanner section 300 is controlled.
Numeral 934 is a drive circuit of a motor by which the polygonal mirror 935 is rotated, and ON and OFF signals for the circuit are supplied from the timing circuit 933.
FIG. 7 is an example of an image exposure system in which a laser beam spot is formed (a laser beam scanner).
Laser beams emitted from the semiconductor laser 931 are incident on the polygonal mirror 935 through mirrors 942 and 943. Laser beams are deflected by the polygonal mirror 935, and irradiate the surface of a photoreceptor drum 130 (a recording medium) through an f.theta. lens 944 by which a laser beam spot is formed so that the diameter of the beam can be a predetermined value.
Numerals 945 and 946 are cylindrical lenses for tilting angle compensation.
Then, laser beams scan the surface of the photoreceptor drum 130 at a predetermined speed in the predetermined direction a, and thereby exposure is conducted corresponding to the pixel data so that an electrostatic latent image is formed. Next, a toner which is charged reversely to the electrostatic latent image by a conventional structure, adheres to the electrostatic latent image and development is conducted. Then, a recording sheet is superposed onto the toner image, charged reversely to the toner image by a corona charger from the rear side of the recording sheet, and then the toner image is transferred onto the recording sheet. Further, the transferred toner image is fixed onto the recording sheet when the toner image is heated or pressed.
In the modulation circuit 110, a pulse width modulation signal (SPWM) is formed by the method disclosed in, for example, Japanese Patent Publication Open to Public Inspection No. 39974/1987.
That is, a digitalized image signal (pixel data) is converted into an analog signal, and when the converted signal is compared with a periodical pattern signal such as a triangle wave as shown in FIG. 8, the pulse width modulation signal (SPWM) having a pulse width corresponding to the image signal can be obtained.
In the structure disclosed in Japanese Patent Publication Open to Public Inspection No. 39974/1987, fullscale amplitude of the image signal can be contained in an amplitude area of the pattern signal, and in the case where the pattern signal is compared with the image signal, a pulse width modulation signal having a predetermined pulse width can be obtained even when the amplitude of the image signal is maximum or minimum.
Accordingly, in the case where fluctuations caused by noises or the like are generated in the pulse width modulation, the laser can not be controlled to be positively turned off or turned on fully, when amplitude of the image signal is maximum or minimum. Therefore, noises are generated on white background and solid areas, so that image reproducibility is lowered, which is a problem.